We have isolated and characterized the distribution of nucleic acid sequences which serve as markers for a highly oncogenic variant of the mouse mammary tumor virus. This variant causes a high incidence of mammary adenocarcinoma in animals of the C3H/He strains of mice and is nongerm line transmitted. Sequences specific for this virus have been designated as tumor associated sequences (TAS) since they are present in DNA from early occurring mammary tumors in C3H/He mice, while absent in DNA from organs of normal C3H/He mice. Sequences represented in the TAS RNA, while absent in DNA from normal organs of most mice examined, are present as endogenous proviruses in GR mice and in two of five substrains examined of C3H mice. By relaxing the conditions of molecular hybridization, nucleic acid sequences related to, but not identical to, the entire RNA genome of MMTV have been detected in the DNA of all rats examined. Furthermore, certain colonies of Fischer rats, and a cloned Fischer rat cell line were found to contain a second class of viral specific nucleic acid sequences related to the TAS-RNA of MMTV. These constitute the first findings of MMTV-related markers in rats.